Radioimmunotherapy for glioblastoma Abstract: Glioblastoma multiforme (GBM) is the most common and lethal primary brain tumor in adults. Current standard- of-care GBM treatment involving surgery and chemoradiation has very limited efficacy. Immunotherapy has been enthusiastically pursued for GBM treatment, but overall, GBM has thus far responded poorly to current immunotherapies, such as cancer therapeutic vaccines and immune checkpoint blockade (ICB). The underlying causes largely involve both local (in the tumor microenvironment) and systemic immunosuppression, heterogeneous and instable tumor cell subpopulations, and central immune tolerance against GBM-associated vaccines. Neoantigens, which are present solely in tumor cells but not in healthy cells, are attractive vaccine candidates due to their lack of central immune tolerance. Indeed, personalized neoantigen vaccines effectively treated some GBM patients. However, GBM generally has very low neoantigen loads, and the vast majority of neoantigens are poorly immunogenic, both of which hinder the wide clinic application of these vaccines. Combination therapy has enormous potential to address these challenges. Here, we propose to develop a novel radioimmunotherapy for GBM by combining fractionated conformal radiation, neoantigen nanovaccines, and ICB to promote the overall therapy response and prolong survival in a pre-clinical orthotopic GBM model. We will test this radioimmunotherapy in an orthotopic GBM model in syngeneic mice. First, fractionated conformal radiation could potently and precisely kill tumor cells and may also abolish local and systemic immunosuppression. Second, a GBM neoantigen nanovaccine will be developed to promote vaccine delivery into lymphoid tissues and antigen-presenting cells (APCs), thereby potentiating immunogenicity of the neoantigen and eliciting potent and durable GBM-specific T cell responses. Third, ICB can further promote anti-GBM immunity. To this end, we demonstrated before the potent therapeutic efficacy of fractionated conformal radiation in an orthotopic GBM mouse model. Moreover, we developed a platform of clinically promising nanovaccines that are formed in vivo from host albumin and albumin-binding vaccines (AlbiVax). AlbiVax are widely applicable and biocompatible. AlbiVax (1) delivered subunit vaccines to lymph nodes ~100-fold more efficiently than a clinic benchmark, (2) efficiently co-delivered molecular adjuvant and antigens to APCs, (3) enhanced antigen presentation, (4) elicited potent and durable antigen-specific immune responses, and (5) exerted great therapeutic efficacy either alone or together with ICB in multiple murine tumor models. Our albumin-binding moiety was validated in human to have efficient lymph node retention and an excellent safety profile. We have a team of investigators with complementary expertise for this study: Dr. Zhu for cancer nanovaccine and immunotherapy; Dr. Valerie for GBM radiotherapy; Dr. Bos for tumor immunology; Dr. Broaddus for clinical neuro-oncology; and Dr. Yan for preclinical/clinical biostatistics. Overall, we are confident to carry out rigorous pilot studies, and eventually establish this radioimmunotherapy strategy for clinical testing. Page 1